1. Field of the Invention
The present invention relates to an antitheft apparatus for preventing an object to be protected against theft from being stolen.
2. Description of the Related Art
A conventional antitheft apparatuses is so designed as to function when its mode is set to an arming mode, i.e. an alert status where an antitheft function works. For example, in the case of an antitheft apparatus for a vehicle, the antitheft function works by means of a transmitter or the like when a user of the vehicle is leaving it. In the arming mode, an alarm is issued even when the user opens a trunk of the vehicle to get a baggage into or from the trunk. Accordingly, in the case where the user opens the trunk, for example, in order to get a baggage thereinto or therefrom, it is usual that the user once changes over the mode to a disarmed mode, i.e. a non-alert status where the antitheft function is suspended. And when the user desires to change over the mode to the arming mode again, the changeover thereto is carried out by operating the transmitter by the user after closing the trunk.
Japanese Unexamined Patent Publication JP-A 4-78751 (1992) discloses a technique for eliminating such a user's troublesome operation of setting to the arming mode again. An antitheft apparatus for automobile disclosed in the publication is configured such that opening a trunk of an automobile in the arming mode by using a key causes the alert status to be temporarily released, that is, causes temporarily setting to the disarmed mode, and then closing the trunk effects automatically shifting to the alert status. It is thus not necessary to conduct a series of operations, namely, shifting the status to a non-alert status once for opening the trunk and then, shifting the status from the non-alert status to the alert status again subsequent to closure of the trunk, shift the status from the non-alert status to the alert status.
The above-described antitheft apparatus may be provided with a sensor of which output is influenced by a change of vehicle weight, an external force on the vehicle, and vibration of the vehicle. The sensor includes, for example, a vibration sensor for detecting a vibration of a vehicle body; an intrusion sensor for detecting an intrusion into a vehicle interior; and a sensor of which output changes in response to motion of an object placed in the vehicle interior attributable to a vibration generated by a change of vehicle weight, an external force and other causes. In the case where the antitheft apparatus is provided with a sensor as stated above, changing over to the arming mode immediately after closure of the trunk may possibly issue an error alarm because the vehicle body continues to vibrate for a short while even after the closure of the trunk. The conventional antitheft apparatus thus has a problem that a theft action cannot be precisely detected.
Further, the antitheft apparatus may be provided with a tilt sensor for detecting an inclined vehicle body angle. In this case, a vehicle body angle in a vehicle-parked state is set as a default angle. However, resetting of a default angle is necessary after getting a baggage into or from the trunk. Accordingly, in order to secure precise detection of a theft action, the user usually waits for the vehicle body to stop vibrating after the closure of the trunk and then carries out the reset of default angle.
In order to solve the problems described above, JP-A 4-78751 discloses a technique of an antitheft apparatus which is designed to effect automatic shifting to the arming mode after an elapse of a predetermined length of time following the closure of the trunk. This constitution allows the arming mode to be set after the halt of vibration following the closure of the trunk so that the error alarm due to the vibration of vehicle body can be prevented.
The sensor (which hereinafter may be collectively referred to as “security sensor”) of which output is influenced by the vibration of vehicle, such as the tilt sensor, the vibration sensor, and the intrusion sensor, is frequently set as optional elements. The theft action may be thus detected through only an antitheft function which is not influenced by the vibration of vehicle, such as a door courtesy switch, a hood switch, or a trunk switch. In such a case, the setting of the arming mode immediately after the closure of the trunk poses no problem. In order to define timing of setting the arming mode after the closure of the trunk, there are two options: the first option is to preset either of an antitheft apparatus having the security sensor and an antitheft apparatus having no security sensor; and the second option is that only the antitheft apparatus having the security sensor is preset and during duration of a predetermined length of time following the closure of the trunk, a user needs to tolerate the disarmed mode. The first option requires manufacture of two antitheft apparatuses, thus resulting in increase of production cost. The second option cannot eliminate unnecessary disarmed mode for a predetermined length of time after the closure of the trunk, thus causing a problem with security for such a period.